As for customary dealings, the governing principle is to focus on the underlying purposes, and that for several reasons:

1. Induction. A comprehensive survey shows that the Lawgiver aims at securing people’s interests, and that rulings in the area of custom follow those interests wherever they lead. Thus a single act may be prohibited when it carries no benefit but permitted when a benefit is present. To exchange one dirham for another on deferred payment is impermissible when it is a sale, yet it is lawful when it takes the form of a loan. Selling fresh dates for dried ones is barred when it amounts to nothing more than usury and uncertainty, but it is allowed when an overriding benefit exists. Nothing comparable is found in the sphere of worship as clearly as it is in customary matters.
   The Qurʾan says, “In legal retribution there is life for you” (2:179) and, “Do not consume one another’s wealth unjustly” (2:188), along with many other verses too numerous to list, all of which point to the consideration of people’s welfare.
2. The Lawgiver has been expansive in disclosing the effective causes behind rulings in the field of custom, unlike the field of worship, where—by consensus—the opposite holds true.
3. Attention to such meanings was already known during the intervals between messengers; rational people relied on it, and their interests were thereby served.
   Once it is clear that meanings and purposes predominate in customary matters, then whenever an element of devotion is introduced, one must stand firmly by the explicit text: for example, the bridal dower required in marriage, slaughtering edible animals in the prescribed place, the fixed shares in inheritance, the set number of months for divorce waiting-periods, and the like.